Jitter measurement device and jitter measurement method

ABSTRACT

An orthogonal signal generating unit converts a signal to be measured into two orthogonal signals which are two signals whose phases are orthogonal to one another. An instantaneous phase calculating unit calculates an instantaneous phase based on the two orthogonal signals within a range between a lower limit phase value set in advance and an upper limit phase value set in advance. A differential value detecting unit detects a differential value of the instantaneous phase. A differential value correcting unit corrects the differential value, and outputs a corrected differential value when the differential value of the instantaneous phase is over the range dependent on the lower limit phase value and the upper limit phase value. An offset component eliminating unit eliminates an offset component included in the corrected differential value from the corrected differential value output from the differential value correcting unit, and outputs a differential value from which the offset component has been eliminated. An integration unit determines a jitter amount of the signal to be measured by integrating the differential value which is output from the offset component eliminating unit, and from which the offset component has been eliminated.

TECHNICAL FIELD

The present invention relates to a jitter measuring apparatus and ajitter measuring method in which a digital computing technology forenabling measurement for a long time without reducing measurementresolution is used in a jitter measuring apparatus and a jittermeasuring method for measuring jitter (phase noise) which has an effecton a signal to be measured, such as a digital signal, used fortransmission of information in a communication network.

BACKGROUND ART

In a data transmission system, when a large jitter arises in atransmission signal, the signal cannot be correctly transmitted.

Therefore, it is necessary to measure the jitter which this type of datatransmission system and the equipment constituting the system generate.

That is, in a digital transmission path for transmitting digitalsignals, the transmission path is extended by repeaters which reproduceand output digital signals. However, in using such repeaters, when phasefluctuation (jitter) in the input signal becomes large, the originalsignal cannot be reproduced.

Therefore, with respect to the maximum permissible level of jitter in aninterface of a digital transmission path, the limit is prescribed invarious international standards.

For example, with respect to the synchronous digital transmissionnetwork called Synchronous Optical Network/Synchronous Digital Hierarchy(SONET/SDH), the maximum permissible level of jitter is prescribed inthe following non-Patent document 1.

Non-Patent document 1: ITU-T Recommendations G.825 (March 2000).Therefore, in order to verify whether or not the jitter generated in theinterface of this type of transmission path satisfies theabove-described maximum permissible level, it is necessary to measurethe jitter amount in advance.

As a conventional jitter measuring apparatus used for such an object,for example, as described in the following Patent document 1, an analogsystem jitter measuring apparatus in which, due to the phase of a signalto be measured (clock signal) being synchronized by using a phase-lockedloop (PLL) circuit, a signal corresponding to the phase shift isdetected as jitter has been known.

Patent document 1: Jpn. Pat. Appln. KOKAI Publication No. 2001-133492.In such an analog system jitter measuring apparatus, there is theproblem that dispersion is brought about in a measured result of ajitter due to dispersion in the characteristics of parts used for thePLL circuit, and the reproducibility of a measured result of a jitterdeteriorates due to environmental variation in temperature and the like.

Further, in an analog system jitter measuring apparatus as describedabove, there is the problem that a measurement range of jitter islimited by a linear operation range of a phase detector (PD) and avoltage controlled oscillator (VCO) which configuration the PLL circuit.

Therefore, in an analog system jitter measuring apparatus as describedabove, because the measurement resolution deteriorates when asensitivity of a PD or a VCO is reduced in order to broaden themeasurement range of jitter, there is a problem in the point that a highresolution and a broad measurement range cannot be compatible.

As a technology to solve these problems, a digital system jittermeasuring apparatus and measuring method which carry out a detection ofa phase error by arithmetic processing are proposed in the followingPatent document 2.

Patent document 2: U.S. Pat. No. 6,621,860. FIG. 5 shows a configurationof a digital system jitter measuring apparatus 10 based on Patentdocument 2.

In this jitter measuring apparatus 10, by sampling a signal to bemeasured C by an analog-to-digital converter 11, the signal to bemeasured C is converted into a digital signal sequence x(n), and thisdigital signal sequence x(n) is input to an orthogonal signal generatingunit 12.

Here, provided that a frequency of a signal to be measured is fc, anamplitude is Ac, a sampling frequency of analog-to-digital conversion isfs, and an initial phase is θc, and a jitter is φ(n), the signalsequence x(n) can be expressed as follows.x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)]  (1)

Where, (n=0,1,2, . . . )

The orthogonal signal generating unit 12 includes a Hilbert transformer,and transforms the signal sequence x(n) of the signal to be measured Cinto two signals I(n), Q(n) whose phases are orthogonal to one another,and outputs these two orthogonal signals I(n), Q(n) to an instantaneousphase calculating unit 13.

Here, the two orthogonal signals I(n), Q(n) are expressed as followswith respect to the above-described signal sequence x(n).I(n)=x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)]Q(n)=Ac·sin [2π(fc/fs)n+θc+φ(n)]  (2)

The instantaneous phase calculating unit 13 calculates an instantaneousphase Θ(n) determined by the two orthogonal signals I(n), Q(n) outputfrom the orthogonal signal generating unit 12, by the followingoperation, and outputs this instantaneous phase Θ(n) to a discontinuitycorrecting unit 14.Θ(n)=tan⁻¹ [Q(n)/I(n)]  (3)

Here, when the two orthogonal signals are expressed by the above formula(2), the instantaneous phase Θ(n) is expressed as follows.$\begin{matrix}\begin{matrix}{{\Theta(n)} = {\tan^{- 1}\left\lbrack {{Q(n)}/{I(n)}} \right\rbrack}} \\\left. {= {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}}} \right\rbrack\end{matrix} & (4)\end{matrix}$

Here, the instantaneous phase Θ(n) determined by the operation of tan⁻¹[Q(n)/I(n)) is limited to a range from −π to π, and for an increase inn, as shown in FIG. 6A, a variation is repeated in which after theinstantaneous phase Θ(n) increases from a value close to an initialphase θc up to nearly π while receiving a fluctuation by the jitter, theinstantaneous phase Θ(n) varies to nearly −π discontinuously, andincreases up to nearly π again.

As described in FIG. 6B, the discontinuity correcting unit 14 correctsthe instantaneous phase Θ(n) which is output from the instantaneousphase calculating unit 13, and which varies discontinuously as describedabove into an instantaneous phase Θ(n) having continuity, and outputsthe corrected instantaneous phase Θ(n) to a linear phase eliminatingunit 15.

The linear phase eliminating unit 15 presumes a phase component2π(fc/fs)n linearly increasing with respect to an increase in n and aninitial phase θc in the corrected instantaneous phases θ(n) output fromthe discontinuity correcting unit 14, and determines a jitter componentφ(n) as shown in FIG. 6C by subtracting the sum of those as a linearphase component L(n) from the instantaneous phases θ(n), and outputsthis jitter component φ(n) to a jitter amount detecting unit 16.

The jitter amount detecting unit 16 detects a jitter amount of thesignal to be measured C based on the jitter component φ(n) output fromthe linear phase eliminating unit 15.

This jitter amount is a maximum amplitude (p-p value) or aroot-mean-square value (rms) of the jitter component φ(n), or anamplitude probability distribution (histogram), a spectrum valueobtained by an FFT operation, or the like, and may be one of or acombination of those.

In this way, by means of a digital system jitter measuring apparatuswhich measures a jitter in a signal to be measured by numeric arithmeticprocessing, as compared with an analog system jitter measuring apparatusdescribed above, there is no reduction in an measuring accuracy ofjitter due to dispersion in the characteristics of parts or anenvironmental variation, and it is possible to measure a jitter withhigh reproducibility, and a broad measurement range and a highmeasurement resolution can be compatible by making a number ofarithmetic bits large.

However, in a technique, as the digital system jitter measuringapparatus 10 described above, in which a jitter component φ(n) isdetermined by subtracting a linear phase component L(n) from aninstantaneous phase θ(n), because the instantaneous phase θ(n) and thelinear phase component L(n) which are signal components diverging astime passes are handled with, there is the problem that a maximummeasurement time for jitter measurement is limited.

That is, this is because the instantaneous phase θ(n) having continuityand the linear phase component L(n) described above increase and divergeas a measurement time passes.

Accordingly, in the digital system jitter measuring apparatus 10described above, because a maximum measurement time for jittermeasurement is limited, there is a problem in the point that jittermeasurement for a long time over the maximum measurement time cannot becarried out.

Specifically, the maximum measurement time for jitter measurement islimited according to a number of bits at the time of operating theinstantaneous phase θ(n) which the discontinuity correcting unit 14 canoutput.

For example, when the number of bits at the time of operating theinstantaneous phase θ(n) is set to 40 bits, provided that 2π(fc/fs) isexpressed by 16 bits in a linear phase component included in theinstantaneous phase Θ(n)L(n)=2π(fc/fs)n+θc,n is limited to a range which the remaining 24 bits can take.

In this case, given that a sampling frequency fs is 100 MHz, because themaximum measurement time for jitter measurement is limited to about0.167 seconds which is (the maximum value of n)/fs, it is impossible tocarry out jitter measurement for a time longer than it (for example, 10seconds or more).

As described in the above-described Patent-document 2, this isoriginally derived from a technique which has been developed in orderfor the digital system jitter measuring apparatus 10 described above toclear a test time of about 0.1 seconds allocated per test item in a testof a very large scale integration (VLSI).

That is, the reason for that the maximum measurement time for jittermeasurement is limited to about 0.167 seconds in the digital systemjitter measuring apparatus 10 described above depends on that jittermeasurement over a long time (for example, 10 seconds or more) is notsupposed in this jitter measuring apparatus 10.

However, in jitter measurement in a communication equipment used for adigital system and a network, and the like, as described in theabove-described non-Patent document 1, for example, a measurement timefor verifying the level of jitter in an interface of a transmission pathis prescribed to 60 seconds.

Given that the above-described n is expressed by 33 bits, measurementfor 60 seconds due to this prescription is possible. However, inaccordance therewith, because needs 49 bits as a result of adding 16bits expressing the above-described 2π(fc/fs) are necessary as thenumber of bits of θ(n), there is a problem in the point that a hardwareamount of an apparatus executing operations is made larger than the caseof 40 bits described above.

Then, given that 2π(fc/fs) is expressed by 7 bits, and n is expressed by33 bits, because 40 bits is sufficient as the number of bits of θ(n),jitter measurement for 60 seconds is possible as the hardware amount isthe same as in the case of 40 bits.

However, in this case, there is a problem in the point that a phaseaccuracy (phase resolution) of a measured value deteriorates by anamount of 9 bits more than the above-described case of 49 bits.

Note that, in some cases, it is requested to carry out jittermeasurement for a long time over several minutes or more, and moreover,one day or more.

Note that, in the case of the digital system jitter measuring apparatus10 described above as well, a time for jitter measurement can beextended by reducing the resolution of the instantaneous phase θ(n).However, because there is a limit in that, and the measurementresolution is reduced, there is a problem in the point that a highresolution and a long time measurement cannot be compatible.

DISCLOSURE OF INVENTION

An object of the present invention is to solve these problems, and toprovide a jitter measuring apparatus and a jitter measuring method inwhich a high resolution and long time measurement are compatible, andjitter measurement can be highly accurately carried out substantiallywithout limiting a maximum measurement time.

In order to achieve the above object, according to a first aspect of thepresent invention, there is provided a jitter measuring apparatuscomprising:

an orthogonal signal generating unit (22) which converts a signal to bemeasured into two orthogonal signals which are two signals whose phasesare orthogonal to one another;

an instantaneous phase calculating unit (23) which calculates aninstantaneous phase based on the two orthogonal signals converted by theorthogonal signal generating unit within a range between a lower limitphase value set in advance and an upper limit phase value set inadvance;

a differential value detecting unit (24) which detects a differentialvalue of the instantaneous phase calculated by the instantaneous phasecalculating unit;

a differential value correcting unit (25) which corrects thedifferential value of the instantaneous phase, and which outputs acorrected differential value when the differential value of theinstantaneous phase detected by the differential value detecting unit isover the range dependent on the lower limit phase value and the upperlimit phase value;

an offset component eliminating unit (26) which eliminates an offsetcomponent included in the corrected differential value from thecorrected differential value output by the differential value correctingunit, and which outputs a differential value from which the offsetcomponent has been eliminated; and

an integration unit (27) which determines a jitter amount of the signalto be measured by integrating the differential value which is output bythe offset component eliminating unit, and from which the offsetcomponent has been eliminated.

In order to achieve the above object, according to a second aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the first aspect, wherein

a lower limit and an upper limit of the range dependent on the lowerlimit phase value and the upper limit phase value are respectively equalto or approximately equal to the lower limit phase value and the upperlimit phase value.

In order to achieve the above object, according to a third aspect of thepresent invention, there is provided the jitter measuring apparatusaccording to the first or second aspect, wherein

the differential value correcting unit has

a discontinuous point detecting unit (25 a) which detects thedifferential value of the instantaneous phase as a discontinuous pointof the differential value of the instantaneous phase when thedifferential value of the instantaneous phase detected by thedifferential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and

a continuity insuring unit (25 b) which insures continuity of thedifferential value of the instantaneous phase by correcting adiscontinuous point of the differential value of the instantaneous phasedetected by the discontinuous point detecting unit with respect to thedifferential value of the instantaneous phase detected by thedifferential value detecting unit, and outputting the correcteddifferential value.

In order to achieve the above object, according to a fourth aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the first or second aspect, wherein

the differential value correcting unit has

a discontinuous point detecting unit (25 a) which detects thedifferential value of the instantaneous phase as a discontinuous pointof the differential value of the instantaneous phase when thedifferential value of the instantaneous phase detected by thedifferential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and

a discontinuous point eliminating unit (25 c) which eliminates thediscontinuous point of the differential value of the instantaneous phasedetected by the discontinuous point detecting unit with respect to thedifferential value of the instantaneous phase detected by thedifferential value detecting unit and outputs the eliminateddifferential value.

In order to achieve the above object, according to a fifth aspect of thepresent invention, there is provided the jitter measuring apparatusaccording to the first or second aspect, wherein

the differential value correcting unit has

a discontinuous point detecting unit (25 a) which detects thedifferential value of the instantaneous phase as a discontinuous pointof the differential value of the instantaneous phase when thedifferential value of the instantaneous phase detected by thedifferential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and

a discontinuous point interpolating unit (25 b) which substantiallyinsures continuity of the differential value of the instantaneous phaseby eliminating the discontinuous point of the differential value of theinstantaneous phase detected by the discontinuous point detecting unitwith respect to the differential value of the instantaneous phasedetected by the differential value detecting unit and interpolating theeliminated portion and outputting the interpolated differential value.

In order to achieve the above object, according to a sixth aspect of thepresent invention, there is provided the jitter measuring apparatusaccording to the first aspect, wherein

the offset component eliminating unit has

a memory (26 a) in which a value determined by an operation of 2π(fc/fs)showing the offset component is stored in advance when a frequency fc ofthe signal to be measured and a sampling frequency fs for sampling thesignal to be measured have been already known, and

a subtraction unit (26 b) which subtracts the value showing the offsetcomponent stored in the memory from the corrected differential valueoutput by the differential value correcting unit.

In order to achieve the above object, according to a seventh aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the first aspect, wherein

the offset component eliminating unit has

a mean value calculating unit (26 c) which determines a mean value ofthe corrected differential value output by the differential valuecorrecting unit as an offset component in advance when a frequency fc ofthe signal to be measured and a sampling frequency fs for sampling thesignal to be measured have been unknown, and

a subtraction unit (26 b) which subtracts the mean value of thecorrected differential value serving as the offset component determinedby the mean value calculating unit from the corrected differential valueoutput by the differential value correcting unit.

In order to achieve the above object, according to an eighth aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the first aspect, wherein,

when a lower limit frequency fj of a jitter component which is an objectto be detected is designated, the offset component eliminating unitincludes a high pass filter (26 d) which has a frequency equal to orapproximately equal to the lower limit frequency fj of the jittercomponent which is the object to be detected as a cutoff frequency foreliminating the offset component from the corrected differential valueoutput by the differential value correcting unit.

In order to achieve the above object, according to a ninth aspect of thepresent invention, there is provided the jitter measuring apparatusaccording to the first aspect, wherein,

given that a digital signal sequence of the signal to be measured isx(n), a frequency and an amplitude of the signal to be measured arerespectively fc and Ac, a sampling frequency for sampling the signal tobe measured is fs, and an initial phase and a jitter of the signal to bemeasured are respectively θc and φ(n), n=0,1,2, . . . , and when I(n),Q(n) serving as the two orthogonal signals are respectively expressed byI(n)=x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)],Q(n)=Ac·sin [2π(fc/fs)n+θc+φ(n)],and Θ(n) serving as the instantaneous phase is expressed by$\begin{matrix}{{\Theta(n)} = {\tan^{- 1}\left\lbrack {{Q(n)}/{I(n)}} \right\rbrack}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}}},}\end{matrix}$the instantaneous phase calculating unit calculates Θ(n) serving as theinstantaneous phase determined by an operation of the tan⁻¹ [Q(n)/I(n)]within a range from −π to π, or −π/2 to π/2 as a range between the lowerlimit phase value set in advance and the upper limit phase value set inadvance.

In order to achieve the above object, according to a tenth aspect of thepresent invention, there is provided the jitter measuring apparatusaccording to the ninth aspect, wherein

the differential value detecting unit calculates ΔΘ(n) serving as thedifferential value of the instantaneous phase calculated by theinstantaneous phase calculating unit, by an operation of $\begin{matrix}{{{\Delta\Theta}(n)} = {{\Theta(n)} - {\Theta\left( {n - 1} \right)}}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}} + {\phi(n)} - {\phi\left( {n - 1} \right)}}}\quad}\end{matrix}$here, 2π(fc/fs) is a constant and an offset component).

In order to achieve the above object, according to an eleventh aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the tenth aspect, wherein

the differential value correcting unit carries out arithmetic processingof Δθ(n) = ΔΘ(n) (−π ≦ ΔΘ(n) ≦ π), Δθ(n) = ΔΘ(n) + 2π (−π > ΔΘ(n)),Δθ(n) = ΔΘ(n) − 2π (ΔΘ(n) > π) or Δθ(n) = ΔΘ(n) (−π/2 ≦ ΔΘ(n) ≦ π/2),Δθ(n) = ΔΘ(n) + π (−π/2 > ΔΘ(n)), Δθ(n) = ΔΘ(n) − π (ΔΘ(n) > π/2)with respect to ΔΘ(n) serving as the differential value of theinstantaneous phase in order to calculate Δθ(n) serving as the correcteddifferential value corrected so as to insure continuity by correctingthe discontinuous point of ΔΘ(n) serving as the differential value ofthe instantaneous phase calculated by the instantaneous phasecalculating unit.

In order to achieve the above object, according to a twelfth aspect ofthe present invention, there is provided the jitter measuring apparatusaccording to the eleventh aspect, wherein the offset componenteliminating unit eliminates the offset component 2π(fc/fs) from Δθ(n)serving as the corrected differential value corrected so as to insurethe continuity by the differential value correcting unit, and outputsΔφ(n)=φ(n)−φ(n−1)as Δφ(n) serving as the differential value from which the offsetcomponent has been eliminated to the integration unit.

In order to achieve the above object, according to a thirteenth aspectof the present invention, there is provided the jitter measuringapparatus according to the twelfth aspect, wherein

the integration unit

carries out a following integration

U(n)=ΣΔφ(i) (Where, the symbol Σ denotes the sum total of i=0,1,2, . . ., n, and here, provided that U(n) is theoretically expanded, it is madeto beU(n)=[φ(0)−φ(−1)]+[φ(1)−φ(0)]+[φ(2)−φ(1)+ . . . +[φ(n)−φ(n−1)]

=φ(n)−φ(n−1) with respect to Δφ(n) serving as the differential valuewhich is output by the offset component eliminating unit, and from whichthe offset component has been eliminated, and

here, given that φ(−1)=0, an integrated result U(n) is to express ajitter component φ(n) of the signal to be measured, and

outputs the integrated result U(n) as the jitter component φ(n) of thesignal to be measured.

In order to achieve the above object, according to a fourteenth aspectof the present invention, there is provided a jitter measuring methodcomprising:

a step of converting a signal to be measured into two orthogonal signalswhich are two signals whose phases are orthogonal to one another;

a step of calculating an instantaneous phase based on the two orthogonalsignals converted by the step of converting into the two orthogonalsignals within a range between a lower limit phase value set in advanceand an upper limit phase value set in advance;

a step of detecting a differential value of the instantaneous phasecalculated by the step of calculating the instantaneous phase;

a step of correcting the differential value of the instantaneous phase,and outputting a corrected differential value when the differentialvalue of the instantaneous phase detected by the step of detecting thedifferential value of the instantaneous phase is over the rangedependent on the lower limit phase value and the upper limit phasevalue;

a step of eliminating an offset component included in the correcteddifferential value from the corrected differential value output by thestep of correcting the differential value of the instantaneous phase,and outputting a differential value from which the offset component hasbeen eliminated; and

a step of determining a jitter amount of the signal to be measured byintegrating the differential value which is output by the step ofeliminating the offset component, and from which the offset componenthas been eliminated.

In order to achieve the above object, according to a fifteenth aspect ofthe present invention, there is provided the jitter measuring methodaccording to the fourteenth aspect, wherein

a lower limit and an upper limit of the range dependent on the lowerlimit phase value and the upper limit phase value are respectively equalto or approximately equal to the lower limit phase value and the upperlimit phase value.

In order to achieve the above object, according to a sixteenth aspect ofthe present invention, there is provided the jitter measuring methodaccording to the fourteenth or fifteenth aspect, wherein

the step of correcting the differential value of the instantaneous phasehas

a step of detecting the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe step of detecting the differential value of the instantaneous phaseis over the range dependent on the lower limit phase value and the upperlimit phase value, and

a step of insuring continuity of the differential value of theinstantaneous phase by correcting the discontinuous point of thedifferential value of the instantaneous phase detected by the step ofdetecting the discontinuous point of the differential value of theinstantaneous phase with respect to the differential value of theinstantaneous phase detected by the step of detecting the differentialvalue of the instantaneous phase, and outputting the correcteddifferential value.

In order to achieve the above object, according to a seventeenth aspectof the present invention, there is provided the jitter measuring methodaccording to the fourteenth or fifteenth aspect, wherein

the step of correcting the differential value of the instantaneous phasehas

a step of detecting the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe step of detecting the differential value of the instantaneous phaseis over the range dependent on the lower limit phase value and the upperlimit phase value, and

a step of eliminating the discontinuous point of the differential valueof the instantaneous phase detected by the step of detecting thediscontinuous point of the differential value of the instantaneous phasewith respect to the differential value of the instantaneous phasedetected by the step of detecting the differential value of theinstantaneous phase, and outputting the eliminated differential value.

In order to achieve the above object, according to an eighteenth aspectof the present invention, there is provided the jitter measuring methodaccording to the fourteenth or fifteenth aspect, wherein

the step of correcting the differential value of the instantaneous phasehas

a step of detecting the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe step of detecting the differential value of the instantaneous phaseis over the range dependent on the lower limit phase value and the upperlimit phase value, and

a step of substantially insuring continuity of the differential value ofthe instantaneous phase by eliminating the discontinuous point of thedifferential value of the instantaneous phase detected by the step ofdetecting the discontinuous point of the differential value of theinstantaneous phase with respect to the differential value of theinstantaneous phase detected by the step of detecting the differentialvalue of the instantaneous phase, and interpolating the eliminatedportion, and outputting the interpolated differential value.

In order to achieve the above object, according to a nineteenth aspectof the present invention, there is provided the jitter measuring methodaccording to the fourteenth aspect, wherein

the step of eliminating the offset component has

a step of storing a value determined by an operation of 2π(fc/fs)showing the offset component in a memory in advance when a frequency fcof the signal to be measured and a sampling frequency fs for samplingthe signal to be measured have been already known, and

a step of subtracting the value showing the offset component stored inthe memory from the corrected differential value output by the step ofcorrecting the differential value of the instantaneous phase.

In order to achieve the above object, according to a twentieth aspect ofthe present invention, there is provided the jitter measuring methodaccording to the fourteenth aspect, wherein

the step of eliminating the offset component has

a step of determining a mean value of the corrected differential valueoutput by the step of correcting the differential value of theinstantaneous phase as an offset component in advance when a frequencyfc of the signal to be measured and a sampling frequency fs for samplingthe signal to be measured have been unknown, and

a step of subtracting the mean value of the corrected differential valueserving as the offset component determined by the step of determiningthe mean value of the corrected differential value, from the correcteddifferential value output by the step of correcting the differential ofthe instantaneous phase.

In order to achieve the above object, according to a twenty-first aspectof the present invention, there is provided the jitter measuring methodaccording to the fourteenth aspect, wherein,

when a lower limit frequency fj of a jitter component which is an objectto be detected is designated, the step of eliminating the offsetcomponent uses a high pass filter which has a frequency equal to orapproximately equal to the lower limit frequency fj of the jittercomponent which is the object to be detected as a cutoff frequency foreliminating the offset component from the corrected differential valueoutput by the step of correcting the differential value of theinstantaneous phase.

In order to achieve the above object, according to a twenty-secondaspect of the present invention, there is provided the jitter measuringmethod according to the fourteenth aspect, wherein,

given that a digital signal sequence of the signal to be measured isx(n), a frequency and an amplitude of the signal to be measured arerespectively fc and Ac, a sampling frequency for sampling the signal tobe measured is fs, and an initial phase and a jitter of the signal to bemeasured are respectively θc and φ(n), n=0,1,2, . . . , and when I(n),Q(n) serving as the two orthogonal signals are respectively expressed byI(n)=x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)],Q(n)=Ac·sin [2π(fc/fs)n+θc+φ(n)],and Θ(n) serving as the instantaneous phase is expressed by$\begin{matrix}{{\Theta(n)} = {\tan^{- 1}\left\lbrack {{Q(n)}/{I(n)}} \right\rbrack}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}}},}\end{matrix}$the step of calculating the instantaneous phase calculates Θ(n) servingas the instantaneous phase determined by an operation of the tan⁻¹[Q(n)/I(n)] within a range from −π to π, or −π/2 to π/2 as a rangebetween the lower limit phase value set in advance and the upper limitphase value set in advance.

In order to achieve the above object, according to a twenty-third aspectof the present invention, there is provided the jitter measuring methodaccording to the twenty-second aspect, wherein

the step of detecting the differential value of the instantaneous phasecalculates ΔΘ(n) serving as the differential value of the instantaneousphase calculated by the step of calculating the instantaneous phase, byan operation of $\begin{matrix}{{{\Delta\Theta}(n)} = {{\Theta(n)} - {\Theta\left( {n - 1} \right)}}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}} + {\phi(n)} - {\phi\left( {n - 1} \right)}}}\quad}\end{matrix}$here, 2π(fc/fs) is a constant and an offset component).

In order to achieve the above object, according to a twenty-fourthaspect of the present invention, there is provided the jitter measuringmethod according to the twenty-third aspect, wherein

the step of correcting the differential value of the instantaneous phasecarries out arithmetic processing of Δθ(n) = ΔΘ(n) (−π ≦ ΔΘ(n) ≦ π),Δθ(n) = ΔΘ(n) + 2π (−π > ΔΘ(n)), Δθ(n) = ΔΘ(n) − 2π (ΔΘ(n) > π) or Δθ(n)= ΔΘ(n) (−π/2 ≦ ΔΘ(n) ≦ π/2), Δθ(n) = ΔΘ(n) + π (−π/2 > ΔΘ(n)), Δθ(n) =ΔΘ(n) − π (ΔΘ(n) > π/2)with respect to ΔΘ(n) serving as the differential value of theinstantaneous phase in order to calculate Δθ(n) serving as the correcteddifferential value corrected so as to insure continuity by correcting adiscontinuous point of ΔΘ(n) serving as the differential value of theinstantaneous phase calculated by the step of calculating theinstantaneous phase.

In order to achieve the above object, according to a twenty-fifth aspectof the present invention, there is provided the jitter measuring methodaccording to the twenty-fourth aspect, wherein the step of eliminatingthe offset component eliminates the offset component 2π(fc/fs) fromΔθ(n) serving as the corrected differential value corrected so as toinsure the continuity by the step of correcting the differential value,and outputsΔφ(n)=φ(n)−φ(n−1)as Δφ(n) serving as the differential value from which the offsetcomponent has been eliminated.

In order to achieve the above object, according to a twenty-sixth aspectof the present invention, there is provided the jitter measuring methodaccording to the twenty-fifth aspect, wherein

the step of integrating the differential value from which the offsetcomponent has been eliminated

carries out a following integration

U(n)=ΣΔφ(i) (Where, the symbol Σ denotes the sum total of i=0,1,2, . .., n, and here, provided that U(n) is theoretically expanded, it is madeto beU(n)=[φ(0)−φ(−1)]+[φ(1)−φ(0)]+[φ(2)−φ(1)+ . . .+[φ(n)−φ(n−1)]=φ(n)−φ(n−1)with respect to Δφ(n) serving as the differential value which is outputby the step of eliminating the offset component, and from which theoffset component has been eliminated,

and here, given that φ(−1)=0, an integrated result U(n) is to express ajitter component φ(n) of the signal to be measured, and

outputs the integrated result U(n) as the jitter component φ(n) of thesignal to be measured.

In this way, in the jitter measuring apparatus and the jitter measuringmethod according to the present invention, an instantaneous phase isdetermined within a predetermined range based on two orthogonal signalsinto which a signal to be measured is orthogonally transformed, and adifferential value of the instantaneous phase is detected, and when thedifferential value of the instantaneous phase is over the predeterminedrange, a predetermined correction such as, for example, insuring thecontinuity, or the like, is carried out, and an offset component iseliminated from the differential value onto which the predeterminedcorrection such as insuring the continuity or the like has been carriedout, and a jitter component of the signal to be measured is determinedby integrating the differential value from which the offset componenthas been eliminated.

Therefore, in the jitter measuring apparatus and the jitter measuringmethod according to the present invention, as a number of arithmeticbits at the respective portions and the respective stages, a number ofarithmetic bits (for example, 16+1 bits) for obtaining a differentialvalue with a necessary accuracy within a range double the predeterminedrange when an instantaneous phase is determined is sufficient, andmoreover, jitter measurement over a long time which is, for example,several minutes or more is possible without limiting to the number ofarithmetic bits, and substantially, without limiting a maximummeasurement time for jitter measurement.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a configuration of an embodiment of ajitter measuring apparatus to which a jitter measuring apparatus and ajitter measuring method according to the present invention is applied.

FIG. 2A is a block diagram showing one example of a differential valuecorrecting unit in the embodiment of FIG. 1.

FIG. 2B is a block diagram showing another example of the differentialvalue correcting unit in the embodiment of FIG. 1.

FIG. 2C is a block diagram showing yet another example of thedifferential value correcting unit in the embodiment of FIG. 1.

FIG. 3A is a block diagram showing one example of an offset componenteliminating unit in the embodiment of FIG. 1.

FIG. 3B is a block diagram showing another example of the offsetcomponent eliminating unit in the embodiment of FIG. 1.

FIG. 3C is a block diagram showing yet another example of the offsetcomponent eliminating unit in the embodiment of FIG. 1.

FIG. 4A is a signal waveform chart shown in order to explain operationsof the jitter measuring apparatus and the jitter measuring methodaccording to the embodiment of FIG. 1.

FIG. 4B is a signal waveform chart shown in order to explain operationsof the jitter measuring apparatus and the jitter measuring methodaccording to the embodiment of FIG. 1.

FIG. 4C is a signal waveform chart shown in order to explain operationsof the jitter measuring apparatus and the jitter measuring methodaccording to the embodiment of FIG. 1.

FIG. 4D is a signal waveform chart shown in order to explain operationsof the jitter measuring apparatus and the jitter measuring methodaccording to the embodiment of FIG. 1.

FIG. 4E is a signal waveform chart shown in order to explain operationsof the jitter measuring apparatus and the jitter measuring methodaccording to the embodiment of FIG. 1.

FIG. 5 is a block diagram showing a configuration of a conventionaljitter measuring apparatus.

FIG. 6A is a signal waveform chart shown in order to explain operationsof the conventional jitter measuring apparatus.

FIG. 6B is a signal waveform chart shown in order to explain operationsof the conventional jitter measuring apparatus.

FIG. 6C is a signal waveform chart shown in order to explain operationsof the conventional jitter measuring apparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of a jitter measuring apparatus and a jittermeasuring method according to the present invention will be describedbased on the drawings.

FIG. 1 shows a configuration of an embodiment of a jitter measuringapparatus 20 to which the jitter measuring apparatus and the jittermeasuring method according to the present invention are applied.

As the basic configuration, the jitter measuring apparatus according tothe present invention includes: an orthogonal signal generating unit 22which converts a signal to be measured C into two orthogonal signalsI(n), Q(n) which are two signals whose phases are orthogonal to oneanother; an instantaneous phase calculating unit 23 which calculates aninstantaneous phase Θ(n) based on the two orthogonal signals I(n), Q(n)converted by the orthogonal signal generating unit 22 within a rangebetween a lower limit phase value set in advance (for example, −π or−π/2) and an upper limit phase value set in advance (for example, π orπ/2); a differential value detecting unit 24 which detects adifferential value ΔΘ(n) of the instantaneous phase Θ(n) calculated bythe instantaneous phase calculating unit 23; a differential valuecorrecting unit 25 which corrects the differential value ΔΘ(n) of theinstantaneous phase, and which outputs a corrected differential valueΔθ(n) when the differential value ΔΘ(n) of the instantaneous phasedetected by the differential value correcting unit 24 is over a range(for example, from −π to π, or from −π/2 to π/2) dependent on the lowerlimit phase value (for example, −π or −π/2) and the upper limit phasevalue (for example, π or π/2); an offset component eliminating unit 26which eliminates an offset component Dc included in the correcteddifferential value Δθ(n) from the corrected differential value Δθ(n)output by the differential value correcting unit 25, and which outputs adifferential value Δφ(n) from which the offset component Dc iseliminated; and an integration unit 27 which determines a jitter amountφ(n) of the signal to be measured C by integrating the differentialvalue Δφ(n) which is output by the offset component eliminating unit 26,and from which the offset component Dc has been eliminated.

As the basic configuration, the jitter measuring method according to thepresent invention includes: a step of converting a signal to be measuredinto two orthogonal signals I(n), Q(n) which are two signals whosephases are orthogonal to one another; a step of calculating aninstantaneous phase Θ(n) based on the two orthogonal signals I(n), Q(n)converted by the step of converting into two orthogonal signals I(n),Q(n) within a range between a lower limit phase value set in advance(for example, −π or −π/2) and an upper limit phase value set in advance(for example, π or π/2); a step of detecting a differential value ΔΘ(n)of the instantaneous phase Θ(n) calculated by the step of calculating aninstantaneous phase Θ(n); a step of correcting the differential valueΔΘ(n) of the instantaneous phase, and outputting a correcteddifferential value Δθ(n) when a differential value ΔΘ(n) of theinstantaneous phase θ(n) detected by the step of detecting adifferential value ΔΘ(n) of the instantaneous phase θ(n) is over a range(for example, from −π to π, or from −π/2 to π/2) dependent on the lowerlimit phase value (for example, −π or −π/2) and the upper limit phasevalue (for example, π or π/2); a step of eliminating an offset componentDc included in the corrected differential value Δθ(n) from the correcteddifferential value Δθ(n) output by the step of correcting a differentialvalue ΔΘ(n) of the instantaneous phase, and outputting a differentialvalue Δφ(n) from which the offset component Dc is eliminated; and a stepof determining a jitter amount φ(n) of the signal to be measured C byintegrating the differential value Δφ(n) which is output by the step ofeliminating an offset component Dc, and from which the offset componentDc has been eliminated.

In the above description, a case is included in which the lower limitand the upper limit of the range dependent on the lower limit phasevalue and the upper limit phase value are respectively made equal to orapproximately equal to the lower limit phase value and the upper limitphase value.

Specifically, in the jitter measuring apparatus 20 according to thepresent embodiment shown in FIG. 1, an analog-to-digital converter 21and the orthogonal signal generating unit 22 are equal to theanalog-to-digital converter 11 and the orthogonal signal generating unit12 of the conventional jitter measuring apparatus 10 described above.

That is, the analog-to-digital converter 21 converts an analog signal tobe measured C into a digital signal sequence x(n) by sampling thereof,and outputs the digital signal sequence x(n)to the orthogonal signalgenerating unit 22.

Here, in the same way as the case of the conventional jitter measuringapparatus 10 described above, provided that a frequency and an amplitudeof the signal to be measured C are respectively fc and Ac, a samplingfrequency of the analog-to-digital converter 21 is fs, and an initialphase and a jitter of the signal to be measured C are respectivelyθcφ(n), the signal sequence x(n) can be expressed by the formula (1)described above.

The orthogonal signal generating unit 22 includes a Hilbert transformer,and converts the signal sequence x(n) of the signal to be measured Coutput from the analog-to-digital converter 21 into two orthogonalsignals I(n), Q(n) expressed by the formula (2) described above, andoutputs these two orthogonal signals I(n), Q(n) to the instantaneousphase calculating unit 23.

The instantaneous phase calculating unit 23 determines an instantaneousphase Θ(n) determined based on the aforementioned two orthogonal signalsI(n), Q(n) output from the orthogonal signal generating unit 22, withina range (for example, a predetermined range from −π to π) between alower limit phase value set in advance (for example, −π ) and an upperlimit phase value set in advance (for example, π) by the formula (3)described above, and outputs this instantaneous phase Θ(n) to thedifferential value detecting unit 24.

The differential value detecting unit 24 detects a differential valueΔΘ(n) of the instantaneous phase Θ(n) output from the differential valuedetecting unit 24, and outputs this differential value ΔΘ(n) of theinstantaneous phase Θ(n) to the differential value correcting unit 25.ΔΘ(n)=Θ(n)−Θ(n−1)   (5)

Where, Θ(n−1)=0

Here, a theoretical value of ΔΘ(n) is expressed as follows.$\begin{matrix}\begin{matrix}{{{\Delta\Theta}(n)} = {{\Theta(n)} - {\Theta\left( {n - 1} \right)}}} \\{{= \left\lbrack {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}} \right\rbrack}\quad} \\{{= \left\lbrack {{2\pi\left( {{fc}/{fs}} \right)\left( {n - 1} \right)}\quad + {\theta\quad c} + {\phi\left( {n - 1} \right)}} \right\rbrack}\quad} \\{{= {{2\pi\left( {{fc}/{fs}} \right)} + {\phi(n)} - {\phi\left( {n - 1} \right)}}}\quad}\end{matrix} & (6)\end{matrix}$

2π(fc/fs) in the above formula (6) is a constant (such as an offsetcomponent).

The differential value correcting unit 25 detects a discontinuous pointof the differential value ΔΘ(n) of the instantaneous phase Θ(n) bycarrying out the following arithmetic processing with respect to thedifferential value ΔΘ(n) of the instantaneous phase Θ(n) output by thedifferential value detecting unit 24, and outputs a differential valueΔθ(n) to which a predetermined correction with respect to thediscontinuous point is applied, for example, which has been corrected soas to insure the continuity, to the offset component eliminating unit26. Δθ(n) = ΔΘ(n) (−π ≦ ΔΘ(n) ≦ π) (7) Δθ(n) = ΔΘ(n) + 2π (−π > ΔΘ(n))Δθ(n) = ΔΘ(n) − 2π (Δθ(n) > π)

FIG. 2A is a block diagram showing one example of the differential valuecorrecting unit 25.

This differential value correcting unit 25 has a discontinuous pointdetecting unit 25 a which detects the differential value of theinstantaneous phase as a discontinuous point of the differential valueof the instantaneous phase when the differential value of theinstantaneous phase detected by the differential value detecting unit 24is over the range dependent on the lower limit phase value and the upperlimit phase value, and a continuity insuring unit 25 b which insures thecontinuity of the differential value of the instantaneous phase bycorrecting the discontinuous point of the differential value of theinstantaneous phase detected by the discontinuous point detecting unit25 a, and by outputting the differential value of the instantaneousphase detected by the differential value detecting unit 24.

FIG. 2B is a block diagram showing another example of the differentialvalue correcting unit 25.

This differential value correcting unit 25 has the discontinuous pointdetecting unit 25 a which detects the differential value of theinstantaneous phase as a discontinuous point of the differential valueof the instantaneous phase when the differential value of theinstantaneous phase detected by the differential value detecting unit 24is over the range dependent on the lower limit phase value and the upperlimit phase value, and a discontinuous point eliminating unit 25 c whicheliminates the discontinuous point of the differential value of theinstantaneous phase detected by the discontinuous point detecting unit25 a with respect to the differential value of the instantaneous phasedetected by the differential value detecting unit 24, and outputs theeliminated differential value.

FIG. 2C is a block diagram showing yet another example of thedifferential value correcting unit 25.

This differential value correcting unit 25 has the discontinuous pointdetecting unit 25 a which detects the differential value of theinstantaneous phase as a discontinuous point of the differential valueof the instantaneous phase when the differential value of theinstantaneous phase detected by the differential value detecting unit 24is over the range dependent on the lower limit phase value and the upperlimit phase value, and a discontinuous point interpolating unit 25 dwhich substantially insures the continuity of the differential value ofthe instantaneous phase by eliminating the discontinuous point of thedifferential value of the instantaneous phase detected by thediscontinuous point detecting unit 25 a, with respect to thedifferential value of the instantaneous phase detected by thedifferential value detecting unit 24, and by interpolating theeliminated portion and outputting the interpolated differential value.

The offset component eliminating unit 26 eliminates an offset componentDc from a differential value Δθ(n) corrected, for example, so as toinsure the continuity of the differential value by the differentialvalue correcting unit 25, and outputs the differential value Δφ(n) fromwhich the offset component Dc has been eliminated to the integrationunit 27.

Here, because the theoretical value of the offset component Dc is2π(fc/fs), a theoretical value of the differential value Δφ(n) is to beΔφ(n)=φ(n)−φ(n−1)   (8)

FIG. 3A is a block diagram showing one example of the offset componenteliminating unit 26.

This offset component eliminating unit 26 has a memory 26 a in which avalue determined by an operation of 2π(fc/fs) showing the offsetcomponent is stored in advance of jitter measurement when a frequency fcof the signal to be measured C and a sampling frequency fs for samplingthe signal to be measured C have been already known, and a subtractionunit 26 b which subtracts the value showing the offset component storedin this memory 26 a from the corrected differential value output by thedifferential value correcting unit 25 at the time of jitter measurement.

FIG. 3B is a block diagram showing another example of the offsetcomponent eliminating unit 26.

This offset component eliminating unit 26 has a mean value calculatingunit 26 c which determines a mean value of the corrected differentialvalue output by the differential value correcting unit 25 as an offsetcomponent in advance of jitter measurement when a frequency fc of thesignal to be measured C and a sampling frequency fs for sampling thesignal to be measured C have been unknown, and the subtraction unit 26 bwhich subtracts the mean value of the corrected differential value asthe offset component determined by the mean value calculating unit 26 cfrom the corrected differential value output by the differential valuecorrecting unit 25 at the time of jitter measurement.

FIG. 3C is a block diagram showing yet another example of the offsetcomponent eliminating unit 26.

This offset component eliminating unit 26 is configured so as to includea high pass filter 26 d which has a frequency equal to or approximatelyequal to the lower limit frequency fj of the jitter component which isthe object to be detected as a cutoff frequency for eliminating theoffset component from the corrected differential value output from thedifferential value correcting unit 25 when a lower limit frequency fj ofa jitter component which is an object to be detected is designated inadvance.

The integration unit 27 carries out the following integration withrespect to the differential value Δφ(n) which is output by the offsetcomponent eliminating unit 26, and from which the offset component Dchas been eliminated, and outputs the integrated result U(n) as a jittercomponent φ(n) of the signal to be measured C to the jitter amountdetecting unit 28.U(n)=ΣΔφ(i)   (9)

Where, the symbol Σ denotes the sum total of i=0,1,2, . . . , n.

Provided that U(n) is theoretically expanded, it is $\begin{matrix}{{U(n)} = {{\left\lbrack {{\phi(0)} - {\phi\left( {- 1} \right)}} \right\rbrack\quad + \left\lbrack {{\phi(1)} - {\phi(0)}} \right\rbrack\quad + \left\lbrack {{\phi(2)} - {\phi(1)}} \right\rbrack\quad + \ldots\quad + \left\lbrack {{\phi(n)} - {\phi\left( {n - 1} \right)}} \right\rbrack}\quad = {{\phi(n)} - {\phi\left( {n - 1} \right)}}}} & \quad\end{matrix}$

Here, given that φ(n−1)=0, the integrated result U(n) expresses a jittercomponent φ(n) of the signal to be measured C.

The jitter amount detecting unit 28 determines, with respect to a jittercomponent φ(n) output from the integration unit 27, a maximum amplitude(p-p value), a root-mean-square value (rms), an amplitude probabilitydistribution (histogram) a spectrum by an FFT, or the like of the jittercomponent φ(n), in the same way as the jitter amount detecting unit 16of the conventional jitter measuring apparatus 10 described above, andoutputs those on an unillustrated indicator or the like.

Next, operations of the jitter measuring apparatus 20 having theabove-described configuration will be described.

Due to the signal to be measured C being converted into a digital signalsequence x(n) by sampling thereof by the analog-to-digital converter 21,and being input to the orthogonal signal generating unit 22, two signalsI(n), Q(n) orthogonal to one another are obtained with respect to thatsignal sequence x(n).

These two orthogonal signals I(n), Q(n) are input to the instantaneousphase calculating unit 23, and an instantaneous phase Θ(n) determined bythe two orthogonal signals I(n), Q(n) is determined within a range (forexample, a predetermined range from −π to π) between a lower limit phasevalue set in advance (for example, −π) and an upper limit phase valueset in advance (for example, π).

As shown in FIG. 4A, this instantaneous phase θ(n) repeats a variationin which after increasing from nearly an initial phase θc up to nearly πas n(=1, 2, . . . ) increases while receiving a fluctuation by jitter,the instantaneous phase Θ(n) varies to nearly −π discontinuously, andincreases again.

Further, although not illustrated, in contrast to the case of FIG. 4A,there are cases in which the instantaneous phase Θ(n) variesdiscontinuously to nearly π again under the effect of jitter immediatelyafter the varying discontinuously from nearly π to nearly −π.

On the other hand, at the differential value detecting unit 24 which hasreceived the instantaneous phase Θ(n), as shown in FIG. 4B, thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n) issuccessively calculated.

This differential value ΔΘ(n) with respect to the instantaneous phaseΘ(n) is to have a discontinuous point discontinuously varying to a valueless than −π when the instantaneous phase Θ(n) has discontinuouslyvaried, for example, from nearly π to nearly −π.

Further, although not illustrated, in contrast to the case of FIG. 4B,when the instantaneous phase Θ(n) discontinuously varies from nearly −πto nearly π, the differential value ΔΘ(n) with respect to theinstantaneous phase Θ(n) is to have a discontinuous pointdiscontinuously varying to a value greater than π.

Such a discontinuous point of the differential value ΔΘ(n) with respectto the instantaneous phase Θ(n) is corrected by the differential valuecorrecting unit 25, for example, as shown in FIG. 4C.

FIG. 4C is a case in which the differential value correcting unit 25 hasa configuration shown in FIG. 2A described above.

That is, at the differential value correcting unit 25 shown in FIG. 2A,when the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is within a predetermined range (for example, −π≦ΔΘ(n)≦π),because a discontinuous point is not detected by the discontinuous pointdetecting unit 25 a, the differential value ΔΘ(n) with respect to theinstantaneous phase Θ(n) is output as a corrected differential valueΔθ(n) as is from the continuity insuring unit 25 b.

Then, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly π to nearly−π, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made smaller than −π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and due to thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)being corrected so as to add by 2π corresponding to a width of thepredetermined range by the continuity insuring unit 25 b on the basisthereof, the continuity of the differential value Δθ(n) with respect tothe instantaneous phase Θ(n) is insured and output from the continuityinsuring unit 25 b.

Then, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly −π to nearlyπ, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made larger than π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and due to thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)being corrected so as to subtract by 2π by the continuity insuring unit25 b on the basis thereof, the continuity of the differential valueΔθ(n) with respect to the instantaneous phase Θ(n) is insured and outputfrom the continuity insuring unit 25 b.

Further, in the case of the differential value correcting unit 25 shownin FIG. 2B, when the differential value ΔΘ(n) with respect to theinstantaneous phase Θ(n) is within a predetermined range (for example,−π≦ΔΘ(n)≦π), because a discontinuous point is not detected by thediscontinuous point detecting unit 25 a, the differential value ΔΘ(n)with respect to the instantaneous phase Θ(n) is output as a correcteddifferential value Δθ(n) as is from the discontinuous point eliminatingunit 25 c.

Then, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly π to nearly−π, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made smaller than −π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and a differentialvalue Δθ(n) corrected such that the differential value ΔΘ(n) of theinstantaneous phase Θ(n) is eliminated as a discontinuous point by thediscontinuous point eliminating unit 25 c on the basis thereof is outputfrom the discontinuous point eliminating unit 25 c.

Then, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly −π to nearlyπ, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made larger than π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and a differentialvalue Δθ(n) corrected such that the differential value ΔΘ(n) at thattime is eliminated as a discontinuous point by the discontinuous pointeliminating unit 25 c on the basis thereof is output from thediscontinuous point eliminating unit 25 c.

In this way, when there is the discontinuous point in a differentialvalue ΔΘ(n), the differential value correcting unit 25 shown in FIG. 2Bis applied to jitter measurement in which such that it does not muchmatter even if the differential value Δθ(n) output from thediscontinuous point eliminating unit 25 c of the discontinuity pointcorrecting unit 25 is made partially to be a so-called toothless stateby eliminating the discontinuous point by the discontinuous pointeliminating unit 25 c.

Further, in the case of the differential value correcting unit 25 shownin FIG. 2C, when the differential value ΔΘ(n) with respect to theinstantaneous phase Θ(n) is within a predetermined range (for example,−π≦ΔΘ(n)≦π), because a discontinuous point is not detected by thediscontinuous point detecting unit 25 a, the differential value ΔΘ(n)with respect to the instantaneous phase Θ(n) is output as is as acorrected differential value Δθ(n) from the discontinuous pointinterpolating unit 25 b.

Then, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly π to nearly−π, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made smaller than −π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and due to thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)being eliminated as a discontinuous point by the discontinuous pointinterpolating unit 25 b on the basis thereof, and due to the eliminatedportion being corrected so as to linearly interpolate on the basis of,for example, the front and rear differential values ΔΘ(n−1) and ΔΘ(n+1)with respect to the instantaneous phase Θ(n) at that time,substantially, the continuity of the differential value Δθ(n) is insuredand output from the discontinuous point interpolating unit 25 b.

Further, at this differential value correcting unit 25, when theinstantaneous phase Θ(n) discontinuously varies from nearly −π to nearlyπ, and the differential value ΔΘ(n) with respect to the instantaneousphase Θ(n) is made larger than π, it is detected as a discontinuouspoint by the discontinuous point detecting unit 25 a, and due to thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)being eliminated as a discontinuous point by the discontinuous pointinterpolating unit 25 b on the basis thereof, and due to the eliminatedportion being corrected so as to linearly interpolate on the basis of,for example, the differential values ΔΘ(n−1) and ΔΘ(n+1) with respect tothe instantaneous phase Θ(n) before and after that time, substantially,the continuity of the differential value Δθ(n) is insured and outputfrom the discontinuous point interpolating unit 25 b.

Note that, the differential value correcting unit 25 shown in FIG. 2A,as a case in which the lower limit and the upper limit of the rangedependent on the lower limit phase value and the upper limit phase valueare made equal to or approximately equal to the lower limit phase valueand the upper limit phase value, when the range dependent on the lowerlimit phase value −π and the upper limit phase value π is over ½ of thedifference (2π) between the lower limit phase value −π and the upperlimit phase value π (i.e., π), may have the discontinuous pointdetecting unit 25 a which detects a discontinuous point of thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)detected at the differential value detecting unit 24, and the continuityinsuring unit 25 b which insures the continuity of the differentialvalue Δθ(n) by outputting a differential value Δθ(n) in which thediscontinuous point of the differential value ΔΘ(n) of the instantaneousphase Θ(n) detected by the discontinuous point detecting unit 25 a hasbeen corrected, with respect to the differential value ΔΘ(n) of theinstantaneous phase Θ(n) detected by the differential value detectingunit 24.

Further, the differential value correcting unit 25 shown in FIG. 2B, asa case in which the lower limit and the upper limit of the rangedependent on the lower limit phase value and the upper limit phase valueare made equal to or approximately equal to the lower limit phase valueand the upper limit phase value, when the range dependent on the lowerlimit phase value −π and the upper limit phase value π is over ½ of thedifference (2π) between the lower limit phase value −π and the upperlimit phase value π (i.e., π), may have the discontinuous pointdetecting unit 25 a which detects a discontinuous point of thedifferential value ΔΘ(n) with respect to the instantaneous phase Θ(n)detected at the differential value detecting unit 24, and thediscontinuous point eliminating unit 25 c which eliminates thediscontinuous point of the differential value ΔΘ(n) of the instantaneousphase Θ(n) detected by the discontinuous point detecting unit 25 a withrespect to the differential value ΔΘ(n) of the instantaneous phase Θ(n)detected by the differential value detecting unit 24 and outputs theeliminated differential value ΔΘ(n).

Further, the differential value correcting unit 25 shown in FIG. 2C, asa case in which the lower limit and the upper limit of the rangedependent on the lower limit phase value and the upper limit phase valueare made equal to or approximately equal to the lower limit phase valueand the upper limit phase value, when the range dependent on the lowerlimit phase value −π and the upper limit phase value π is over ½ of thedifference (2π) between the lower limit phase value −π and the upperlimit phase value π (i.e., π), may have the discontinuous pointdetecting unit 25 a which detects a discontinuous point of thedifferential value ΔΘ(n) of the instantaneous phase Θ(n) detected at thedifferential value detecting unit 24, and the discontinuous pointinterpolating unit (25 d) which substantially insures the continuity ofthe differential value Δθ(n) by eliminating the discontinuous point ofthe differential value ΔΘ(n) of the instantaneous phase Θ(n) detected bythe discontinuous point detecting unit 25 a, with respect to thedifferential value ΔΘ(n) of the instantaneous phase Θ(n) detected by thedifferential value detecting unit 24 and by interpolating the eliminatedportion by, for example, a linear interpolation as described above, orthe like, and by outputting the interpolated differential value ΔΘ(n).

Next, the offset component eliminating unit 26, as described above,outputs a differential value Δφ(n) from which the offset component Dc iseliminated to the integration unit 27, as shown in FIG. 4D, byeliminating an offset component Dc corresponding to 2π(fc/fs) superposedupon a differential value Δθ(n) corrected, for example, so as to insurethe continuity by the differential value interpolating unit 25.

That is, in the case of the offset component eliminating unit 26 shownin FIG. 3A, provided that a sampling frequency fs of theanalog-to-digital converter 21 and a frequency fc of the signal to bemeasured C have been already known, a value of the offset component2π(fc/fs) is determined and stored in the memory 26 a in advance ofjitter measurement, and at the time of jitter measurement, the offsetcomponent Dc can be eliminated by using a configuration in which astored value in the memory 26 a is subtracted from the differentialvalue Δθ(n) by the subtraction unit 26 b.

Further, in the case of the offset component eliminating unit 26 shownin FIG. 3B, when a frequency fc of the signal to be measured C and asampling frequency fs have been unknown, and a value of the offsetcomponent 2π(fc/fs) is obscure, as shown in 3B, a mean value H of thedifferential value Δθ(n) is determined by the mean value calculatingunit 26 c in advance of jitter measurement, and the offset component Dccan be eliminated by subtracting the mean value H from the differentialvalue Δθ(n) of the instantaneous phase input at the time of jittermeasurement by the subtraction unit 26 b.

Further, in the case of the offset component eliminating unit 26 shownin FIG. 3C, provided that a lower limit frequency fj (for example, 10Hz) of the jitter component which is an object to be detected isdesignated, as shown in FIG. 3C, by using a high-pass filter 26 d havinga frequency equal to or approximately equal to the lower limit frequencyfj of the jitter component which is the object to be detected as acutoff frequency, the offset component Dc can be eliminated from thedifferential value Δθ(n) of the instantaneous phase input at the time ofjitter measurement.

Then, the integration unit 27 determines a jitter component φ(n) of thesignal to be measured C as shown in FIG. 4E by integrating thedifferential value Δφ(n) from which the offset component Dc has beeneliminated by the offset component eliminating unit 26, and outputs thisjitter component φ(n) of the signal to be measured C to the jitteramount detecting unit 28.

The jitter amount detecting unit 28 determines, with respect to thejitter component φ(n) output from the integration unit 27, a maximumamplitude value (p-p value), a root-mean-square value (rms), a histogramor a spectrum thereof, and outputs to indicate those on an indicator orthe like.

As described above, in the jitter measuring apparatus and the jittermeasuring method in the present embodiment, an instantaneous phase Θ(n)determined based on the two orthogonal signals I(n), Q(n) generated fromthe signal to be measured C is determined within a range from −π to π(predetermined range), and a differential value ΔΘ(n) of theinstantaneous phase Θ(n) is calculated, and a predetermined correctionsuch as, for example, insuring the continuity, or the like is carriedout with respect to the differential value ΔΘ(n) of the instantaneousphase Θ(n), and an offset component Dc is eliminated from thedifferential value Δθ(n) corrected, for example, so as to insure thecontinuity, and a jitter component φ(n) of the signal to be measured Cis determined by integrating the differential value Δφ(n) from which theoffset component Dc has been eliminated.

Therefore, as the number of arithmetic bits at the respective portionsand the respective stages, the number of bits (for example, 16+1 bits)for obtaining the differential value ΔΘ(n) from −2π to 2π (a range whichis double the predetermined range at the time of determining aninstantaneous phase) with a necessary accuracy is sufficient, and jittermeasurement over a long time, for example, several minutes or more ismade possible without limiting to the number of arithmetic bits, andmoreover, substantially, without limiting the maximum measurement timefor jitter measurement.

Note that, in the above-described embodiment, the instantaneous phaseΘ(n) is determined within a range from −90 to π. However, this range isnot limited from −π to π, and may be arbitrarily set according to amagnitude of an estimated jitter.

For example, in a case of measuring a little jitter, the instantaneousphase Θ(n) may be determined within a range from −π/2 to π/2.

In this case, in the above-described formula (7) is made to be Δθ(n) =ΔΘ(n) (−π/2 ≦ ΔΘ(n) ≦ π/2) (10) Δθ(n) = ΔΘ(n) + π (−π/2 > ΔΘ(n)) Δθ(n) =ΔΘ(n) − π (ΔΘ(n) > π/2)

Further, in the above-described embodiment, the case in which the signalto be measured C is an analog signal was described. However, when ajitter of a signal to be measured which has been digitized is measured,it is recommended that the analog-to-digital converter 21 be omitted,and the digitized signal to be measured be directly input to theorthogonal signal generating unit 22.

Further, in the above-described embodiment, the case in which therespective portions and the respective stages are realized by a hardwareconfiguration was described. However, it is possible to realize bysoftware structures according to various computers including CPUs.

In addition thereto, with respect to the above-described embodiment, itgoes without saying that various modifications and applications arepossible within a range which does not deviate from the gist of thepresent invention.

Accordingly, as described above in detail, according to the presentinvention, a jitter measuring apparatus and a jitter measuring methodcan be provided in which the problems which the above-describedconventional art has are solved, and a high resolution and a long timemeasurement are made compatible, and jitter measurement can be highlyaccurately carried out substantially without limiting the maximummeasurement time.

1. A jitter measuring apparatus comprising: an orthogonal signalgenerating unit which converts a signal to be measured into twoorthogonal signals which are two signals whose phases are orthogonal toone another; an instantaneous phase calculating unit which calculates aninstantaneous phase based on the two orthogonal signals converted by theorthogonal signal generating unit within a range between a lower limitphase value set in advance and an upper limit phase value set inadvance; a differential value detecting unit which detects adifferential value of the instantaneous phase calculated by theinstantaneous phase calculating unit; a differential value correctingunit which corrects the differential value of the instantaneous phase,and which outputs a corrected differential value when the differentialvalue of the instantaneous phase detected by the differential valuedetecting unit is over the range dependent on the lower limit phasevalue and the upper limit phase value; an offset component eliminatingunit which eliminates an offset component included in the correcteddifferential value from the corrected differential value output by thedifferential value correcting unit, and which outputs a differentialvalue from which the offset component has been eliminated; and anintegration unit which determines a jitter amount of the signal to bemeasured by integrating the differential value which is output by theoffset component eliminating unit, and from which the offset componenthas been eliminated.
 2. The jitter measuring apparatus according toclaim 1, wherein a lower limit and an upper limit of the range dependenton the lower limit phase value and the upper limit phase value arerespectively equal to or approximately equal to the lower limit phasevalue and the upper limit phase value.
 3. The jitter measuring apparatusaccording to claim 1 or 2, wherein the differential value correctingunit has a discontinuous point detecting unit which detects thedifferential value of the instantaneous phase as a discontinuous pointof the differential value of the instantaneous phase when thedifferential value of the instantaneous phase detected by thedifferential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and acontinuity insuring unit which insures continuity of the differentialvalue of the instantaneous phase by correcting a discontinuous point ofthe differential value of the instantaneous phase detected by thediscontinuous point detecting unit with respect to the differentialvalue of the instantaneous phase detected by the differential valuedetecting unit, and outputting the corrected differential value.
 4. Thejitter measuring apparatus according to claim 1 or 2, wherein thedifferential value correcting unit has a discontinuous point detectingunit which detects the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe differential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and adiscontinuous point eliminating unit which eliminates the discontinuouspoint of the differential value of the instantaneous phase detected bythe discontinuous point detecting unit with respect to the differentialvalue of the instantaneous phase detected by the differential valuedetecting unit and outputs the eliminated differential value.
 5. Thejitter measuring apparatus according to claim 1 or 2, wherein thedifferential value correcting unit has a discontinuous point detectingunit which detects the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe differential value detecting unit is over the range dependent on thelower limit phase value and the upper limit phase value, and adiscontinuous point interpolating unit which substantially insurescontinuity of the differential value of the instantaneous phase byeliminating the discontinuous point of the differential value of theinstantaneous phase detected by the discontinuous point detecting unitwith respect to the differential value of the instantaneous phasedetected by the differential value detecting unit and interpolating theeliminated portion and outputting the interpolated differential value.6. The jitter measuring apparatus according to claim 1, wherein theoffset component eliminating unit has a memory in which a valuedetermined by an operation of 2π(fc/fs) showing the offset component isstored in advance when a frequency fc of the signal to be measured and asampling frequency fs for sampling the signal to be measured have beenalready known, and a subtraction unit which subtracts the value showingthe offset component stored in the memory from the correcteddifferential value output by the differential value correcting unit. 7.The jitter measuring apparatus according to claim 1, wherein the offsetcomponent eliminating unit has a mean value calculating unit whichdetermines a mean value of the corrected differential value output bythe differential value correcting unit as an offset component in advancewhen a frequency fc of the signal to be measured and a samplingfrequency fs for sampling the signal to be measured have been unknown,and a subtraction unit which subtracts the mean value of the correcteddifferential value serving as the offset component determined by themean value calculating unit from the corrected differential value outputby the differential value correcting unit.
 8. The jitter measuringapparatus according to claim 1, wherein, when a lower limit frequency fjof a jitter component which is an object to be detected is designated,the offset component eliminating unit includes a high-pass filter whichhas a frequency equal to or approximately equal to the lower limitfrequency fj of the jitter component which is the object to be detectedas a cutoff frequency for eliminating the offset component from thecorrected differential value output by the differential value correctingunit.
 9. The jitter measuring apparatus according to claim 1, wherein,given that a digital signal sequence of the signal to be measured isx(n), a frequency and an amplitude of the signal to be measured arerespectively fc and Ac, a sampling frequency for sampling the signal tobe measured is fs, and an initial phase and a jitter of the signal to bemeasured are respectively θc and φ(n), n=0,1,2, . . . , and when I(n),Q(n) serving as the two orthogonal signals are respectively expressed byI(n)=x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)],Q(n)=Ac·sin [2π(fc/fs)n+θc+φ(n)], and Θ(n) serving as the instantaneousphase is expressed by $\begin{matrix}{{\Theta(n)} = {\tan^{- 1}\left\lbrack {{Q(n)}/{I(n)}} \right\rbrack}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}}},}\end{matrix}$ the instantaneous phase calculating unit calculates Θ(n)serving as the instantaneous phase determined by an operation of thetan⁻¹ [Q(n)/I(n)] within a range from −π to π, or −π/2 to π/2 as a rangebetween the lower limit phase value set in advance and the upper limitphase value set in advance.
 10. The jitter measuring apparatus accordingto claim 9, wherein the differential value detecting unit calculatesΔΘ(n) serving as the differential value of the instantaneous phasecalculated by the instantaneous phase calculating unit, by an operationofΔΘ(n)=Θ(n)−Θ(n−1) =2π(fc/fs)+φ(n)−φ(n−1) (Where, Θ(n−1)=0, and here,2π(fc/fs) is a constant and an offset component).
 11. The jittermeasuring apparatus according to claim 10, wherein the differentialvalue correcting unit carries out arithmetic processing of Δθ(n) = ΔΘ(n)(−π ≦ ΔΘ(n) ≦ π), Δθ(n) = ΔΘ(n) + 2π (−π > ΔΘ(n)), Δθ(n) = ΔΘ(n) − 2π(ΔΘ(n) > π) or Δθ(n) = ΔΘ(n) (−π/2 ≦ ΔΘ(n) ≦ π/2), Δθ(n) = ΔΘ(n) + π(−π/2 > ΔΘ(n)), Δθ(n) = ΔΘ(n) − π (ΔΘ(n) > π/2)

with respect to ΔΘ(n) serving as the differential value of theinstantaneous phase in order to calculate Δθ(n) serving as the correcteddifferential value corrected so as to insure continuity by correctingthe discontinuous point of ΔΘ(n) serving as the differential value ofthe instantaneous phase calculated by the instantaneous phasecalculating unit.
 12. The jitter measuring apparatus according to claim11, wherein the offset component eliminating unit eliminates the offsetcomponent 2π(fc/fs) from Δθ(n) serving as the corrected differentialvalue corrected so as to insure the continuity by the differential valuecorrecting unit, and outputsΔφ(n)=φ(n)−φ(n−1) as Δφ(n) serving as the differential value from whichthe offset component has been eliminated to the integration unit. 13.The jitter measuring apparatus according to claim 12, wherein theintegration unit carries out a following integration U(n)=ΣΔφ(i) (Where,the symbol Σ denotes the sum total of i=0,1,2, . . . , n, and here,provided that U(n) is theoretically expanded, it is made to beU(n)=[φ(0)−φ(−1)]+[φ(1)−φ(0)+[φ(2)−φ(1)]+ . . . +[φ(n)−φ(n−1)]=φ(n)−φ(n−1) with respect to Δφ(n) serving as the differential valuewhich is output by the offset component eliminating unit, and from whichthe offset component has been eliminated, and here, given that φ(−1)=0,an integrated result U(n) is to express a jitter component φ(n) of thesignal to be measured, and outputs the integrated result U(n) as thejitter component φ(n) of the signal to be measured.
 14. A jittermeasuring method comprising: a step of converting a signal to bemeasured into two orthogonal signals which are two signals whose phasesare orthogonal to one another; a step of calculating an instantaneousphase based on the two orthogonal signals converted by the step ofconverting into the two orthogonal signals within a range between alower limit phase value set in advance and an upper limit phase valueset in advance; a step of detecting a differential value of theinstantaneous phase calculated by the step of calculating theinstantaneous phase; a step of correcting the differential value of theinstantaneous phase, and outputting a corrected differential value whenthe differential value of the instantaneous phase detected by the stepof detecting the differential value of the instantaneous phase is overthe range dependent on the lower limit phase value and the upper limitphase value; a step of eliminating an offset component included in thecorrected differential value from the corrected differential valueoutput by the step of correcting the differential value of theinstantaneous phase, and outputting a differential value from which theoffset component has been eliminated; and a step of determining a jitteramount of the signal to be measured by integrating the differentialvalue which is output by the step of eliminating the offset component,and from which the offset component has been eliminated.
 15. The jittermeasuring method according to claim 14, wherein a lower limit and anupper limit of the range dependent on the lower limit phase value andthe upper limit phase value are respectively equal to or approximatelyequal to the lower limit phase value and the upper limit phase value.16. The jitter measuring method according to claim 14 or 15, wherein thestep of correcting the differential value of the instantaneous phase hasa step of detecting the differential value of the instantaneous phase asa discontinuous point of the differential value of the instantaneousphase when the differential value of the instantaneous phase detected bythe step of detecting the differential value of the instantaneous phaseis over the range dependent on the lower limit phase value and the upperlimit phase value, and a step of insuring continuity of the differentialvalue of the instantaneous phase by correcting the discontinuous pointof the differential value of the instantaneous phase detected by thestep of detecting the discontinuous point of the differential value ofthe instantaneous phase with respect to the differential value of theinstantaneous phase detected by the step of detecting the differentialvalue of the instantaneous phase, and outputting the correcteddifferential value.
 17. The jitter measuring method according to claim14 or 15, wherein the step of correcting the differential value of theinstantaneous phase has a step of detecting the differential value ofthe instantaneous phase as a discontinuous point of the differentialvalue of the instantaneous phase when the differential value of theinstantaneous phase detected by the step of detecting the differentialvalue of the instantaneous phase is over the range dependent on thelower limit phase value and the upper limit phase value, and a step ofeliminating the discontinuous point of the differential value of theinstantaneous phase detected by the step of detecting the discontinuouspoint of the differential value of the instantaneous phase with respectto the differential value of the instantaneous phase detected by thestep of detecting the differential value of the instantaneous phase, andoutputting the eliminated differential value.
 18. The jitter measuringmethod according to claim 14 or 15, wherein the step of correcting thedifferential value of the instantaneous phase has a step of detectingthe differential value of the instantaneous phase as a discontinuouspoint of the differential value of the instantaneous phase when thedifferential value of the instantaneous phase detected by the step ofdetecting the differential value of the instantaneous phase is over therange dependent on the lower limit phase value and the upper limit phasevalue, and a step of substantially insuring continuity of thedifferential value of the instantaneous phase by eliminating thediscontinuous point of the differential value of the instantaneous phasedetected by the step of detecting the discontinuous point of thedifferential value of the instantaneous phase with respect to thedifferential value of the instantaneous phase detected by the step ofdetecting the differential value of the instantaneous phase, andinterpolating the eliminated portion, and outputting the interpolateddifferential value.
 19. The jitter measuring method according to claim14, wherein the step of eliminating the offset component has a step ofstoring a value determined by an operation of 2π(fc/fs) showing theoffset component in a memory in advance when a frequency fc of thesignal to be measured and a sampling frequency fs for sampling thesignal to be measured have been already known, and a step of subtractingthe value showing the offset component stored in the memory from thecorrected differential value output by the step of correcting thedifferential value of the instantaneous phase.
 20. The jitter measuringmethod according to claim 14, wherein the step of eliminating the offsetcomponent has a step of determining a mean value of the correcteddifferential value output by the step of correcting the differentialvalue of the instantaneous phase as an offset component in advance whena frequency fc of the signal to be measured and a sampling frequency fsfor sampling the signal to be measured have been unknown, and a step ofsubtracting the mean value of the corrected differential value servingas the offset component determined by the step of determining the meanvalue of the corrected differential value, from the correcteddifferential value output by the step of correcting the differential ofthe instantaneous phase.
 21. The jitter measuring method according toclaim 14, wherein, when a lower limit frequency fj of a jitter componentwhich is an object to be detected is designated the step of eliminatingthe offset component uses a high-pass filter which has a frequency equalto or approximately equal to the lower limit frequency fj of the jittercomponent which is the object to be detected as a cutoff frequency foreliminating the offset component from the corrected differential valueoutput by the step of correcting the differential value of theinstantaneous phase.
 22. The jitter measuring method according to claim14, wherein, given that a digital signal sequence of the signal to bemeasured is x(n), a frequency and an amplitude of the signal to bemeasured are respectively fc and Ac, a sampling frequency for samplingthe signal to be measured is fs, and an initial phase and a jitter ofthe signal to be measured are respectively θc and φ(n), n=0,1,2, . . . ,and when I(n), Q(n) serving as the two orthogonal signals arerespectively expressed byI(n)=x(n)=Ac·cos [2π(fc/fs)n+θc+φ(n)],Q(n)=Ac·sin [2π(fc/fs)n+θc+φ(n)], and Θ(n) serving as the instantaneousphase is expressed by $\begin{matrix}{{\Theta(n)} = {\tan^{- 1}\left\lbrack {{Q(n)}/{I(n)}} \right\rbrack}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}n} + {\theta\quad c} + {\phi(n)}}},}\end{matrix}$ the step of calculating the instantaneous phase calculatesΘ(n) serving as the instantaneous phase determined by an operation ofthe tan⁻¹ [Q(n)/I(n)) within a range from −π to π, or −π/2 to π/2 as arange between the lower limit phase value set in advance and the upperlimit phase value set in advance.
 23. The jitter measuring methodaccording to claim 22, wherein the step of detecting the differentialvalue of the instantaneous phase calculates ΔΘ(n) serving as thedifferential value of the instantaneous phase calculated by the step ofcalculating the instantaneous phase, by an operation of $\begin{matrix}{{{\Delta\Theta}(n)} = {{\Theta(n)} - {\Theta\left( {n - 1} \right)}}} \\{{= {{2{\pi\left( {{fc}/{fs}} \right)}} + {\phi(n)} - {\phi\left( {n - 1} \right)}}}\quad}\end{matrix}$ here, 2π(fc/fs) is a constant and an offset component).24. The jitter measuring method according to claim 23, wherein the stepof correcting the differential value of the instantaneous phase carriesout arithmetic processing of Δθ(n) = ΔΘ(n) (−π ≦ ΔΘ(n) ≦ π), Δθ(n) =ΔΘ(n) + 2π (−π > ΔΘ(n)), Δθ(n) = ΔΘ(n) − 2π (ΔΘ(n) > π) or Δθ(n) = ΔΘ(n)(−π/2 ≦ ΔΘ(n) ≦ π/2), Δθ(n) = ΔΘ(n) + π (−π/2 > ΔΘ(n)), Δθ(n) = ΔΘ(n) −π (ΔΘ(n) > π/2)

with respect to ΔΘ(n) serving as the differential value of theinstantaneous phase in order to calculate Δθ(n) serving as the correcteddifferential value corrected so as to insure continuity by correcting adiscontinuous point of ΔΘ(n) serving as the differential value of theinstantaneous phase calculated by the step of calculating theinstantaneous phase.
 25. The jitter measuring method according to claim24, wherein the step of eliminating the offset component eliminates theoffset component 2π(fc/fs) from Δθ(n) serving as the correcteddifferential value corrected so as to insure the continuity by the stepof correcting the differential value, and outputsΔφ(n)=φ(n)−φ(n−1) as Δφ(n) serving as the differential value from whichthe offset component has been eliminated.
 26. The jitter measuringmethod according to claim 25, wherein the step of integrating thedifferential value from which the offset component has been eliminatedcarries out a following integration U(n)=ΣΔφ(i) (Where, the symbol Σdenotes the sum total of i=0,1,2, . . . , n, and here, provided thatU(n) is theoretically expanded, it is made to beU(n)=[φ(0)−φ(−1)]+[φ(1)−φ(0)]+[φ(2)−φ(1)+ . . .+[φ(n)−φ(n−1)]=φ(n)−φ(n−1) with respect to Δφ(n) serving as thedifferential value which is output by the step of eliminating the offsetcomponent, and from which the offset component has been eliminated, andhere, given that φ(−1)=0, an integrated result U(n) is to express ajitter component φ(n) of the signal to be measured, and outputs theintegrated result U(n) as the jitter component φ(n) of the signal to bemeasured.